Hydrocarbon gas is frequently processed before storage, transportation through a pipeline or use. Processing removes undesirable components from the gas, such as moisture or sour contaminants.
Processing gas to remove moisture is referred to as dehydration. Hydrocarbon gas containing moisture is typically dehydrated by exposing it to the solvent triethylene glycol. The moisture is removed from the hydrocarbon gas in order to increase the heating value of the gas and to reduce the condensation of free liquid water during transportation or storage. The removal of the moisture also reduces the formation of gas hydrates that foul pipeline equipment.
In a typical glycol dehydration unit, the gas is dehydrated in a gas-liquid contactor, which is typically a tower. The wet gas enters the contactor at the bottom, while the dry gas exits from the top. Inside of the contactor, the gas passes through a shower of glycol solvent. The lean liquid solvent enters the contactor at the top and the rich liquid solvent (solvent containing moisture) exits the contactor from the bottom. The liquid solvent drains down inside of the contactor through a series of internal trays or packing. The gas is forced up through the solvent shower. When the gas physically contacts the liquid solvent, the mass of the water vapor in the gas is transferred to the solvent.
The rich solvent is processed for reuse. Reusing the solvent is desirable for environmental reasons (disposal of the solvent is both difficult and expensive) and also because replacing the solvent is expensive. Processing the solvent removes the moisture wherein the solvent is said to be lean.
Processing gas to remove sour contaminants is referred to as sweetening. Sour gas smells like rotten eggs. The sour contaminants are sulfur compounds (for example, hydrogen sulfide). These sulfur-containing compounds are removed because, when the compounds are combined with water, sulfuric acid is formed. Another contaminant in the gas is carbon dioxide. When the carbon dioxide is combined with water, carbonic acid is formed. Removing these acid forming contaminants is desirable in order to minimize corrosion in the vessels and pipelines used to store and transport the gas.
The gas sweetening process is similar to the dehydration process. The gas is forced upward through a shower of sweetening solvent in a gas-liquid contactor. The sweetening solvent is an amine solvent. The contaminants are removed by the sweetening solvent.
The sweetening solvent is processed for reuse, for the same reasons that the dehydration solvent is processed for reuse. Processing the solvent removes the sour contaminants.
In both the dehydration process and the sweetening process, the gas-liquid contactor requires careful balancing of the physical parameters of the gas and the liquid. When the contactor is in equilibrium, the gas exits out of the top and the rich solvent (the solvent being rich with either moisture or sour contaminants) exits out of the bottom, as described above. Also, when the system is in equilibrium, the amount of gas that is processed is maximized.
One sign that equilibrium is lost is when some of the liquid solvent is carried out of the contactor with the gas. This occurs if the gas rate through the contactor is too high or if the solvent contains relatively high concentrations of foaming contaminants. Such foaming contaminants include well treatment chemicals, liquid hydrocarbons (such as crude oil), corrosion inhibitors, suspended solids and excessive amounts of antifoam chemicals. Foaming is evident when the foam exits the top of the contactor. This is known as “carrying over” or “puking”.
Foaming of the solvent is undesirable because foaming leads to a loss of efficiency of the contactor, causes contamination of the gas with the solvent, and results in the loss of the expensive solvent.
In the prior art, attempts have been made to solve the foaming problem. The prior art treats the solvent by passing it through activated carbon to adsorb the foam causing surfactants. In addition, the solvent is passed through filters to remove small suspended particles. Such particles stabilize the foam once it is formed.
The prior art systems suffered from several problems. The filters require replacement and disposal. Disposal of the used filters can be expensive due to environmental concerns. In addition, the filters themselves are expensive. Filters are also very specialized, being suited only to a narrow range of contaminant types or sizes. It is difficult to select a proper type of filter for the particular foaming contaminant present in the solvent. That is to say that the effectiveness of the filter is dependent on the filter matching the particular type of the foaming contaminant. Typically, the particular type of foaming contaminant is unknown, resulting in guess work as to the particular filter which is to be used.
My U.S. Pat. No. 6,080,320 teaches a method and apparatus for removing foaming contaminants from solvents. I have made improvements to both the method and the apparatus.
One of these improvements provides a much cleaner solvent than ever before obtained, thereby increasing the overall efficiency of the hydrocarbon processing. Foaming contaminants comprise surfactants. In the gas-liquid contactor, a frothing is desired in order to remove the impurities (water, sour contaminants, etc.) from the hydrocarbon gas. This process is known as mass transfer, wherein the impurities are transferred from the hydrocarbon gas to the liquid solvent.
The presence of foaming contaminants reduces the efficiency of the gas-liquid contactor. This is because the foaming contaminants resist the transfer of mass from the gas to the liquid, thereby reducing the quantity of mass that is transferred. The quantity of mass transferred is adversely affected even if the contactor does not exhibit signs of foaming (such as foam production at the top of the contactor or variations in the pressure at the gas outlet). In the prior art, the operator of the contactor detects a foaming problem by detecting foam at the top of the contactor or by a pressure change in the gas outlet. If foam is detected, then the operator adds anti-foaming agents to the solvent.
However, the contactor efficiency is reduced even by a quantity of foaming contaminants that is too small to cause detectable foaming. Thus, even if foaming is undetected by the operator, the efficiency is likely to be relatively low. Furthermore, the addition of anti-foaming agents does not reduce the amount of foaming contaminants. Instead the anti-foaming agents work to reduce the stability of the foam; the foaming contaminants are not tied up. Consequently, the foaming contaminants continue to adversely affect the contactor efficiency.
Another improvement eliminates the need for a compressor to introduce the gas into the apparatus as well as making the introduction of gas independent on the solvent inlet flow. The gas is used to create foam for the removal of the foaming contaminants. Compressors are expensive and can be bulky. An eductor can be used in the solvent input line, but this makes the flow of gas into the apparatus dependent upon the flow of solvent into the apparatus.
Still another improvement modifies the structure of the foam that is generated. Modifying the foam structure results in a drier foam (one that contains less solvent) and bubbles that are more easily broken for ultimate recovery of the liquid component of the foam.
Still another improvement isolates unwanted liquids from the solvent, such as oil. This results in cleaner solvent.